The invention concerns an arrangement of pressure nozzles intended for the treatment of webs, comprising a nozzle box, which has a carrying face placed facing a web, two nozzle slots that blow towards each other being provided in connection with the carrying face, the nozzle slots being placed in the outer part of the space defined by the inner walls and the outer walls or equivalent of the nozzle box.
The nozzle arrangement subject of the invention is intended for contact-free supporting and treatment, such as drying, heating or cooling, of paper webs and other continuous webs.
Apparatuses based on the blowing of gas are used commonly in the manufacture and processing of paper. In the apparatuses meant above, the gas to be blown is guided by means of various nozzle arrangements to one side or to both sides of the web, whereupon the treatment gas is sucked off for reuse or for exhaust, and/or the treatment gas is allowed to be discharged to the sides of the web.
The prior-art apparatuses based on contact-free treatment of the web consist of a number of nozzle boxes, out of whose nozzles a gas flow that supports and dries the web is directed at the web. The prior-art nozzles in said apparatuses can be divided into two groups: nozzles with positive pressure and nozzles with negative pressure, where of the operation of the nozzles with positive pressure is based on the air-cushion principle, and the nozzles with negative pressure attract the web and stabilize the run of the web. The attractive force applied to the web is, as is well known, based on a gas flow field parallel to the web, the field forming a static negative pressure between the web and the carrying face of the nozzle.
Both in nozzles with positive pressure and in those with negative pressure, the so-called Coanda effect is commonly used to guide air in the desired direction.
The force applied to the web from prior-art nozzles with negative pressure is relatively low, for which reason these nozzles cannot be used for the treatment of heavy webs or when the tension of the web is low. Thus, nozzles with negative pressure are, as a rule, used in apparatuses whose length does not exceed 5 m and at both sides of which guide rolls are provided to support the web.
The force applied by positive-pressure nozzles to the web is relatively high. Thus, by means of pressure nozzles it is possible to treat heavy and fully untensioned webs. Most of the prior-art pressure nozzles, however, direct sharp jets substantially perpendicularly to the web, thereby producing an uneven distribution of the heat transfer factor in the longitudinal direction of the web, which frequently results in damage to the quality of the web to be treated.
The blowing out of the prior-art pressure nozzles is also unstable, so that the blow jet may turn, e.g. by the effect of the running of the web, directly from the blow opening into the suction space between the nozzles, thereby causing a lowering of the heat transfer factor and an unstable running of the web.
The prior art discussed above comprising, e.g., the U.S. Pat. No. 3,549,070 as well as from the SE Patents Nos. 341,870 and 352,121. These publications suggest nozzles in which, by means of the Coanda effect, attempts have been made to make the blow jets turn and become parallel to the web. Since the outlet directions of the jets form an angle of 90.degree. relative the web, the jets do not have time to turn and to become parallel to the web they are separated from the guide face of the nozzle. In the paper by D. W. McGlaughine and I. Greber, "Experiments on the Separation of a Fluid Jet from a Curved Surface", The American Society of Mechanical Engineers, Advances in Fluids, 1976, it has also been established that a jet discharged out of a nozzle can, without being separated, follow along with a curved face 45.degree.. . . 70.degree., and a following angle of 70.degree. cannot be exceeded. A separated jet collides against the web and causes a peak of the heat transfer factor at the collision point, whereupon the jet seeks its way into the suction space between the nozzles and allows the space between the nozzle slots in the nozzle, the area of the so-called "carrying face" of the nozzle, to remain untreated, which results in substantially no heat transfer in this area.
With respect to of the prior art most closely related to the present invention, reference is made to the applicant's FI Patent No. 68,723 (equivalent of U.S. Pat. No. 4,247,993), wherein a nozzle with negative pressure is described which is mainly characterized in that, in the direction of flow of the gas, the nozzle slot of the nozzle with negative pressure is placed before the plane of the inlet edge of the curved guide face and that the ratio of the width of the nozzle slot to the curve radius of said guide face is, with the gas flow rates occurring in practice, chosen so that the gas flow is separated from the curved guide face substantially before its trailing edge.
The prior art most closely related to the invention is the applicant's FI Patent 60,261 (equivalent of U.S. Pat. No. 4,384,666), wherein a nozzle with positive pressure is described, wherein it is a novel feature that the nozzle slots are located in such a way relative to the carrying face of the nozzle that the gas jets follow along with the carrying face, without being separated, up to the recess formed between the nozzle slots, that the following angle of the gas jets is at the maximum 70.degree., and that said recess is dimensioned so as to act as a quieting space, wherein the gas jets that flow in opposite directions meet with other and form an air cushion which supports the web and extends over a considerable distance in the direction of running of the web.